High frequency switch

ABSTRACT

Disclosed is a high frequency switch wherein a first switch circuit is connected in series to a first λ/4 signal transmission path connected between an antenna connecting terminal and a transmission terminal. In the first switch circuit, a first λ/4 transmission path and a first parallel resonant circuit, which includes one first PIN diode, are connected in series. In a first inductor of the first parallel resonant circuit, a constant is set so that a resonance frequency of the first parallel resonant circuit and the center frequency of a first antenna switch are the same when the first PIN diode is turned off.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a high frequency switch (RF switch) forswitching between high frequency signals, and more particularly to ahigh frequency switch suitable for use as an antenna switch connected toan antenna, e.g., a TDD (Time Division Duplex) switch or the like.

2. Description of the Related Art

Conventional high frequency switches such as antenna switches include amicrowave switch disclosed in Patent Document 1 and a transmission andreception switching device disclosed in Patent Document 2, for example.

The microwave switch disclosed in Patent Document 1 has PIN diodesinserted in series and parallel in a signal line. Forward currents arepassed through the PIN diodes to turn them on, and the PIN diodes arereversely biased to turn them off, thereby switching between highfrequency signals.

The transmission and reception switching device disclosed in PatentDocument 2 employs a circuit scheme wherein a switch is constructed oftransmission lines and PIN diodes or the like which are connected inseries to the transmission lines, the transmission lines and the PINdiodes being connected parallel to a signal transmission line.

There is also known an example wherein a choke coil is connected for thepurpose of disconnecting bias circuits for PIN diodes at highfrequencies when the PIN diodes are turned off (see Patent Document 3,for example).

Patent Document 1: Japanese Patent No. 2532122

Patent Document 2: Japanese Patent No. 2830319

Patent Document 3: Japanese Patent Publication No. 01-033961

SUMMARY OF THE INVENTION

There are two types of transmission and reception schemes (a firsttransmission and reception scheme and a second transmission andreception scheme) using high frequency switches, as described below.

According to the first transmission and reception scheme, as shown inFIG. 19, a transmission amplifier 108 and an isolator 111 are connectedto a transmission signal line 106 between a transceiver 100 and atransmission and reception antenna 102 (or via a bandpass filter 104),and a reception amplifier 112 is connected to a reception signal line110 between the transceiver 100 and the transmission and receptionantenna 102 (or via the bandpass filter 104). A high frequency switch114 is connected to the junction between the transmission signal line106 and the reception signal line 110.

According to the second transmission and reception scheme, as shown inFIG. 20, a transmission amplifier 108 is connected to a transmissionsignal line 106, and a reception amplifier 112 and a high frequencyswitch 114 are connected to a reception signal line 110. A circulator116 is connected to the junction between the transmission signal line106 and the reception signal line 110.

The above high frequency switch comprises a reflective high frequencyswitch which makes a VSWR (voltage standing wave ratio) infinite fortotal reflection when a switch on the receiver side is turned off. Inthis case, the high frequency switch becomes unstable tending to causeoscillation due to an input mismatch with the reception amplifier. Sucha problem may be solved by inserting another isolator between thereception amplifier and the high frequency switch. However, the isolatorthus inserted is liable to cause a large loss, which makes the receiversensitivity worse at a low level receive signal.

Patent Documents 1 and 2 suffer the following problems:

The microwave switch disclosed in Patent Document 1 switches betweensignals by turning on and off the PIN diodes inserted in the signalline. However, a large insertion loss is caused due to the forwardresidual resistance which is present when the PIN diodes are turned on,and the remnant capacitance and parallel resistance which are presentwhen the PIN diodes are turned off. While the isolation provided whenthe PIN diodes are turned off can be expanded by increasing the numberof PIN diodes that are inserted parallel, the increased number of PINdiodes results in an increased insertion loss. Though the insertion losscaused when the PIN diodes are turned on can be reduced, the isolationis reduced in accordance with the number of PIN diodes.

The transmission and reception switching device disclosed in PatentDocument 2 provides a switch that is constructed of a signaltransmission line, transmission lines and PIN diodes connected in seriesto the transmission line, the transmission lines and the PIN diodesbeing connected parallel to the signal transmission line. In thetransmission and reception switching device, an insertion loss is causeddue to the forward residual resistance of the PIN diodes (which areturned on by the forward bias) when the switching circuit is turned on.Furthermore, the transmission and reception switching device causes thephase characteristic of a λ/4 transmission line to suffer an error dueto the remnant capacitance of the PIN diodes (which are turned off bythe reverse bias) when the switching circuit is turned off. In otherwords, the transmission and reception switching device is problematic inthat the central frequencies at the time the switching circuit is turnedon and off deviate from each other. As a result, the passband providedwhen the switch is turned on and the isolation band provided when theswitch is turned off deviate from each other. Moreover, when theswitching circuit is turned off, the isolation is reduced due to theforward residual resistance of the PIN diodes.

The present invention has been made in view of the above problems. It isan object of the present invention to provide a high frequency switchwhich does not cause the phase characteristic of a λ/4 transmission lineto suffer an error, can bring the passband provided when switch circuitsare turned on and the isolation band provided when the switch circuitsare turned off into conformity with each other, and is capable ofappropriately minimizing the insertion loss caused when the switchcircuits are turned on and maximizing the isolation provided when theswitch circuits are turned off in a band that is used by the highfrequency switch.

Another object of the present invention is to provide a high frequencyswitch which does not cause an input mismatch with a reception amplifiereven when a switched on receiver side is turned off, thereby preventingthe reception amplifier from being unstable in operation.

According to a first aspect of the present invention, a high frequencyswitch having an operating frequency band with a central frequency foand a wavelength λ corresponding to the central frequency fo, comprisesa switch circuit connected parallel to a λ/4 signal transmission linefor transmitting a signal, the switch circuit comprising a λ/4transmission line and a parallel resonant circuit including one or morePIN diodes, the λ/4 transmission line and the parallel resonant circuitbeing connected in series to each other, and the parallel resonantcircuit has a constant established such that the resonant frequencythereof when the one or more PIN diodes is turned off is the same as thecentral frequency fo.

With the above arrangement, the phase characteristic of the λ/4transmission line is free of an error, thereby getting coincidencebetween the passband of the switch circuit when it is turned on and theisolation band thereof when it is turned off. In other words, the highfrequency switch is capable of appropriately minimizing the insertionloss caused when the switch circuit is turned on and maximizing theisolation provided when the switch circuit is turned off in a band thatis used by the high frequency switch. As a result, the loss of atransmission signal caused by the switch circuit is reduced, and anappropriate amount of attenuation at the time the switch circuit isturned off is secured.

In the first aspect, the parallel resonant circuit may include aplurality of PIN diodes. With this arrangement, the insertion loss ofthe switch circuit at the time a signal is transmitted can further bereduced without degrading the isolation at the time the signal is cutoff.

In the first aspect, the λ/4 transmission line may have a characteristicimpedance which is smaller than a characteristic impedance of the λ/4signal transmission line. In this case, the isolation at the time theswitch is turned off can be expanded.

In the first aspect, the λ/4 transmission line may have a characteristicimpedance which is greater than a characteristic impedance of the λ/4signal transmission line. In this case, the insertion loss at the timethe switch is turned on can be minimized.

According to a second aspect of the present invention, a high frequencyswitch includes a first switch circuit connected parallel to a first λ/4signal transmission line for transmitting signal from a transmissionterminal, the first switch circuit comprising a first λ/4 transmissionline and a circuit including one or more first PIN diodes, the first λ/4transmission line and the circuit being connected in series to eachother, and a second switch circuit connected parallel to a second λ/4signal transmission line for receiving signal by a reception terminal,the second switch circuit comprising a second λ/4 transmission line anda circuit including one or more second PIN diodes, the second λ/4transmission line and the circuit being connected in series to eachother, the high frequency switch comprising a third switch circuitconnected parallel to a third λ/4 signal transmission line connected atleast between the reception terminal and the second λ/4 signaltransmission line, the third switch circuit comprising a third λ/4transmission line and a circuit including one or more third PIN diodes,the third λ/4 transmission line and the circuit being connected inseries to each other, and a resistor for forming a terminatingresistance, the resistor connected parallel to the third PIN diode.

With the above arrangement, when the switches on a receiver side (thesecond switch circuit and the third switch circuit) are turned off, noimpedance mismatch occurs with a reception amplifier that is connectedto the reception terminal, thus preventing the reception amplifier frombecoming unstable in operation.

In the second aspect, the high frequency switch may comprise a fourthswitch circuit connected parallel to a fourth λ/4 signal transmissionline connected between the transmission terminal and the first λ/4signal transmission line, the fourth switch circuit comprising a fourthλ/4 transmission line and a circuit including one or more fourth PINdiodes, the fourth λ/4 transmission line and the circuit being connectedin series to each other, and a resistor for forming a terminatingresistance, the resistor connected parallel to the fourth PIN diode.

With the above arrangement, when the switches on a transmitter side (thefirst switch circuit and the fourth switch circuit) are turned off, theterminating resistor is brought into connection to the transmissionterminal. In this case, the impedance of the transmitter side at thetime the switches are turned off is of the value of the terminatingresistor (e.g., 50 ohms), making it possible to achieve impedancematching with other circuits.

In the second aspect, the high frequency switch has an operatingfrequency band with a central frequency fo and a wavelength λcorresponding to the central frequency fo, wherein the first switchcircuit may comprise the first λ/4 transmission line and a parallelresonant circuit including the one or more first PIN diodes, the firstλ/4 transmission line and the parallel resonant circuit being connectedin series to the first λ/4 signal transmission line, the second switchcircuit may comprise the second λ/4 transmission line and a parallelresonant circuit including the one or more second PIN diodes, the secondλ/4 transmission line and the parallel resonant circuit being connectedin series to the second λ/4 signal transmission line, and the thirdswitch circuit may comprise the third λ/4 transmission line and aparallel resonant circuit including the one or more third PIN diodes,the third λ/4 transmission line and the parallel resonant circuit beingconnected in series to the third λ/4 signal transmission line, each ofthe parallel resonant circuits having a constant established such thatthe resonant frequency thereof when the corresponding one of the PINdiodes is turned off is the same as the central frequency fo.

With the above arrangement, the phase characteristic of the λ/4transmission line is free of an error, thereby getting coincidencebetween the passband of the switch circuit when it is turned on and theisolation band thereof when it is turned off. The high frequency switchis capable of appropriately minimizing the insertion loss caused whenthe switch circuit is turned on and maximizing the isolation providedwhen the switch circuit is turned off in a band that is used by the highfrequency switch.

In the second aspect, the first switch circuit may comprise the firstλ/4 transmission line and a parallel resonant circuit including the oneor more first PIN diodes, the first λ/4 transmission line and theparallel resonant circuit being connected in series to the first λ/4signal transmission line, the second switch circuit may comprise thesecond λ/4 transmission line and a parallel resonant circuit includingthe one or more second PIN diodes, the second λ/4 transmission line andthe parallel resonant circuit being connected in series to the secondλ/4 signal transmission line, the third switch circuit may comprise thethird λ/4 transmission line and a parallel resonant circuit includingthe one or more third PIN diodes, the third λ/4 transmission line andthe parallel resonant circuit being connected in series to the third λ/4signal transmission line, and the fourth switch circuit may comprise thefourth λ/4 transmission line and a parallel resonant circuit includingthe one or more fourth PIN diodes, the fourth λ/4 transmission line andthe parallel resonant circuit being connected in series to the fourthλ/4 signal transmission line, each of the parallel resonant circuitshaving a constant established such that the resonant frequency thereofwhen the corresponding one of the PIN diodes is turned off is the sameas the central frequency fo.

In the second aspect, the parallel resonant circuit may include aplurality of PIN diodes. With this arrangement, the insertion loss ofthe switch circuits at the time a signal is transmitted can further bereduced without degrading the isolation at the time the signal is cutoff.

In the second aspect, the λ/4 transmission lines may have acharacteristic impedance which is smaller than a characteristicimpedance of the λ/4 signal transmission lines. In this case, theisolation at the time the switch is turned off can be expanded.

In the second aspect, the λ/4 transmission lines may have acharacteristic impedance which is greater than a characteristicimpedance of the λ/4 signal transmission lines. In this case, theinsertion loss at the time the switch is turned on can be minimized.

With the high frequency switch according to the present invention, asdescribed above, the phase characteristic of the λ/4 transmission lineis free of an error, thereby getting coincidence between the passband ofthe switch circuit when it is turned on and the isolation band thereofwhen it is turned off. The high frequency switch is capable ofappropriately minimizing the insertion loss caused when the switchcircuit is turned on and maximizing the isolation provided when theswitch circuit is turned off in a band that is used by the highfrequency switch.

With the high frequency switch according to the present invention,furthermore, when the switches on the receiver side are turned off, noimpedance mismatch occurs with the reception amplifier, thus preventingthe reception amplifier from becoming unstable in operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing a configuration of a first antennaswitch;

FIG. 2A is a diagram showing an equivalent circuit of a first switchcircuit of the first antenna switch when a first PIN diode is turned on,and FIG. 2B is a diagram showing an equivalent circuit of the firstswitch circuit when the first PIN diode is turned off;

FIG. 3A is a diagram showing an equivalent circuit of the first switchcircuit in the vicinity of a central frequency when the first PIN diodeis turned on, and FIG. 3B is a diagram showing an equivalent circuit ofthe first switch circuit in the vicinity of a central frequency when thefirst PIN diode is turned off;

FIG. 4 is a diagram illustrative of the relationship between input andoutput impedances of a transmission line;

FIG. 5 is a diagram showing an equivalent circuit of the first antennaswitch when the first switch circuit is turned on and a second switchcircuit is turned off;

FIG. 6 is a diagram showing an equivalent circuit of the first antennaswitch when the first switch circuit is turned off and the second switchcircuit is turned on;

FIG. 7 is a circuit diagram showing a configuration of a firstmodification of the first antenna switch;

FIG. 8 is a circuit diagram showing a configuration of a secondmodification of the first antenna switch;

FIG. 9 is a circuit diagram showing a configuration of a second antennaswitch;

FIG. 10 is a circuit diagram showing a configuration of a third antennaswitch;

FIG. 11A is a diagram showing an equivalent circuit of a third switchcircuit of the third antenna switch when a third PIN diode is turned on,and FIG. 11B is a diagram showing an equivalent circuit of the thirdswitch circuit when the third PIN diode is turned off;

FIG. 12 is a diagram showing an equivalent circuit of the third antennaswitch when a first switch circuit is turned on and a second switchcircuit and the third switch circuit are turned off;

FIG. 13 is a circuit diagram showing a configuration of a firstmodification of the third antenna switch;

FIG. 14 is a circuit diagram showing a configuration of a fourthmodification of the third antenna switch;

FIG. 15 is a circuit diagram showing a configuration of a fourth antennaswitch;

FIG. 16 is a circuit diagram showing a configuration of a fifth antennaswitch;

FIG. 17 is a diagram showing an equivalent circuit of the fifth antennaswitch when a first switch circuit and a fourth switch circuit areturned off and a second switch circuit and a third switch circuit areturned on;

FIG. 18 is a circuit diagram showing a configuration of a sixth antennaswitch;

FIG. 19 is a diagram illustrative of a first transmission and receptionscheme using a high frequency switch; and

FIG. 20 is a diagram illustrative of a second transmission and receptionscheme using a high frequency switch.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments wherein a high frequency switch according to the presentinvention is applied, for example, to an antenna switch will bedescribed below with reference to FIGS. 1 through 18. It is assumed thatλ represents a wavelength corresponding to the central frequency of anoperating frequency band of the switch, and refers to a wavelength intransmission lines described below.

As shown in FIG. 1, an antenna switch according to a first embodiment(hereinafter referred to as a first antenna switch 10A) comprises afirst λ/4 signal transmission line 18 a connected between an antennaconnection terminal 14 and a transmission terminal 16, a second λ/4signal transmission line 18 b connected between the antenna connectionterminal 14 and a reception terminal 20, a first switch circuit 22 aconnected parallel to the first λ/4 signal transmission line 18 a, and asecond switch circuit 22 b connected parallel to the second λ/4 signaltransmission line 18 b. Capacitors C1 through C4 are connectedrespectively between the transmission terminal 16 and the first λ/4signal transmission line 18 a, between the first λ/4 signal transmissionline 18 a and the antenna connection terminal 14, between the antennaconnection terminal 14 and the second λ/4 signal transmission line 18 b,and between the second λ/4 signal transmission line 18 b and thereception terminal 20. The capacitors C1 through C4 are capacitors forblocking currents for turning on and off PIN diodes, to be describedlater, and operate as a short circuit at high frequencies.

The first switch circuit 22 a is connected between a signal line betweenthe capacitor C1 and the first λ/4 signal transmission line 18 a and GND(ground). The first switch circuit 22 a comprises a series-connectedcircuit of a first λ/4 transmission line 24 a and a first parallelresonant circuit 26 a which are connected in series to each other at afirst junctional.

The first parallel resonant circuit 26 a comprises a first PIN diode 28a connected between the first junctional and GND, a first inductor 30 aconnected between the first junctional and a first control terminal Tc1,and a first capacitor Ca connected between the first control terminalTc1 and GND. The first capacitor Ca operates as a capacitor for blockingcurrents for turning on and off the first PIN diode 28 a.

To the first control terminal Tc1, there are applied a forward biasvoltage Vc1 for passing a forward current through the first PIN diode 28a to turn on the first PIN diode 28 a and a reverse bias voltage Vc2 forreversely biasing the first PIN diode 28 a to turn off the first PINdiode 28 a.

As with the first switch circuit 22 a described above, the second switchcircuit 22 b is connected between a signal line between the second λ/4signal transmission line 18 b and the capacitor C4 and GND (ground). Thesecond switch circuit 22 b comprises a series-connected circuit of asecond λ/4 transmission line 24 b and a second parallel resonant circuit26 b which are connected in series to each other at a second junctiona2.

The second parallel resonant circuit 26 b comprises a second PIN diode28 b connected between the second junction a2 and GND, a second inductor30 b connected between the second junction a2 and a second controlterminal Tc2, and a second capacitor Cb connected between the secondcontrol terminal Tc2 and GND. The second capacitor Cb operates as acapacitor for blocking currents for turning on and off the second PINdiode 28 b.

To the second control terminal Tc2, there are applied the forward biasvoltage Vc1 for passing a forward current through the second PIN diode28 b to turn on the second PIN diode 28 b and the reverse bias voltageVc2 for reversely biasing the second PIN diode 28 b to turn off thesecond PIN diode 28 b. When the forward bias voltage Vc1 is applied tothe first control terminal Tc1, the reverse bias voltage Vc2 is appliedto the second control terminal Tc2. When the reverse bias voltage Vc2 isapplied to the first control terminal Tc1, the forward bias voltage Vc1is applied to the second control terminal Tc2. The reverse bias voltageVc2 which is applied to the first control terminal Tc1 and the reversebias voltage Vc2 which is applied to the second control terminal Tc2 mayhave different voltage levels.

Circuit operation of the first antenna switch 10A will be describedbelow with reference to FIGS. 2 through 6.

The first switch circuit 22 a will primarily be described below. Whenthe forward bias voltage Vc1 is applied to the first control terminalTc1, the first PIN diode 28 a is turned on. At this time, the firstswitch circuit 22 a is represented by an equivalent circuit shown inFIG. 2A. Specifically, a circuit comprising an inductance La and an ONresistance Ro of the first PIN diode 28 a which are connected parallelto each other is connected in series between the first λ/4 transmissionline 24 a and GND.

Conversely, when the reverse bias voltage Vc2 is applied to the firstcontrol terminal Tc1, the first PIN diode 28 a is turned off. At thistime, the first switch circuit 22 a is represented by an equivalentcircuit shown in FIG. 2B. Specifically, a parallel resonant circuitcomprising an inductance La, a parasitic capacitance Cf due to thedepletion layer of the first PIN diode 28 a, and an parallel resistanceRf of the first PIN diode 28 a which are connected parallel to eachother is connected in series between the first λ/4 transmission line 24a and GND.

In the first antenna switch 10A, the inductance La has a valueestablished such that the central frequency fo of the first antennaswitch 10A and the resonant frequency of the parallel resonant circuitthat is made up of the parasitic capacitance Cf, the parallel resistanceRf, and the inductance La are in agreement with each other.

The ON resistance Ro is generally of about 1 ohm or less. Since the ONresistance Ro can be expressed as Ro<<2πfoLa, the first switch circuit22 a can be represented by an equivalent circuit shown in FIG. 3A in thevicinity of the central frequency fo when the first PIN diode 28 a isturned on, and can be represented by an equivalent circuit shown in FIG.3B in the vicinity of the central frequency fo when the first PIN diode28 a is turned off.

It is assumed that, as shown in FIG. 4, a transmission line z=L isterminated by the load of an impedance Z(L).

If the transmission line has a characteristic impedance Zo, a travellingwave is represented by Ae^(−γz), and a reflected wave is represented byBe^(−γz) (γ indicates a propagation constant), then a voltage V(z) and acurrent I(z) at a reference point z are expressed by the followingequations:V(z)=Ae ^(−γz) +Be ^(γz)I(z)=(A/Zo)e ^(−γz)−(B/Zo)e ^(γz)

Therefore, the impedance Z(L) at z=L is expressed by the followingequation:

$\begin{matrix}{{Z(L)} = {{V(L)}/{I(L)}}} \\{= {{Zo}\left\{ {\left( {{A\;{\mathbb{e}}^{{- \gamma}\; L}} + {B\;{\mathbb{e}}^{\gamma\; L}}} \right)/\left( {{A\;{\mathbb{e}}^{{- \gamma}\; L}} - {B\;{\mathbb{e}}^{\gamma\; L}}} \right)} \right\}}}\end{matrix}$

A reflection coefficient Γ(L) has a relationship expressed by thefollowing equation (a):

$\begin{matrix}\begin{matrix}{{\Gamma(L)} = {\left( {B\;{\mathbb{e}}^{\gamma\; L}} \right)/\left( {A\;{\mathbb{e}}^{{- \gamma}\; L}} \right)}} \\{= {\left( {B/A} \right)\;{\mathbb{e}}^{2\gamma\; L}}} \\{= {\left\{ {{Z(L)} - {Zo}} \right\}/\left\{ {{Z(L)} + {Zo}} \right\}}}\end{matrix} & (a)\end{matrix}$

An impedance Z(0) of the load as seen at z=0 is expressed by thefollowing equation (b):Z(0)=Zo{(A+B)/(A−B)}  (b)

From the equation (a),B/A=[{Z(L)−Zo}/{Z(L)+Zo}]e ^(−2γL)

By substituting this equation into the equation (b), the followingequation (c) is obtained:Z(0)/Zo=[Z(L)+Zo tan hγL]/[Zo+Z(L)tan hγL]  (c)

where γ=α+jβ (α represents an attenuation constant and β a phaseconstant expressed by β=2π/λ).

Since α=0 and γ=jβ for a lossless line, the equation (c) can be modifiedinto the following equation (d):Z(0)/Zo=[Z(L)+jZo tan βL]/[Zo+jZ(L)tan βL]  (d)

By substituting L=λ/4 into the equation (d), the following equation (e)is obtained:Z(0)/Zo=Zo/Z(L)Z(0)=Zo ² /Z(L)  (e)

Inasmuch as Z(L) is a low resistance of about 1 ohm or less when thefirst PIN diode 28 a is turned on, the impedance (in this case, Z(0)) ofthe first λ/4 transmission line 24 a on the signal line side is of alarge value, and the signal line is ideally in an open state, as can beunderstood from the equation (e). Conversely, inasmuch as Z(L) is a highresistance of about 10 k ohms or more when the first PIN diode 28 a isturned off, the impedance (in this case, Z(0)) of the first λ/4transmission line 24 a on the signal line side is of a small value, andthe signal line is ideally in a short-circuited state, as can beunderstood from the equation (e).

The second switch circuit 22 b behaves similarly. When the second PINdiode 28 b is turned on, the impedance of the second λ/4 transmissionline 24 b on the signal line side is of a large value, and the signalline is ideally in an open state. Conversely, when the second PIN diode28 b is turned off, the impedance of the second λ/4 transmission line 24b on the signal line side is of a small value, and the signal line isideally in a short-circuited state.

Therefore, when the forward bias voltage Vc1 is applied to the firstcontrol terminal Tc1, turning on the first PIN diode 28 a, and thereverse bias voltage Vc2 is applied to the second control terminal Tc2,turning off the second PIN diode 28 b, the first antenna switch 10A isrepresented by an equivalent circuit shown in FIG. 5 wherein only thetransmission terminal 16 is connected to the antenna connection terminal14 at high frequencies. A transmission signal Sa supplied to thetransmission terminal 16 is thus transmitted via the antenna connectionterminal 14. In other words, a first signal line 34 a from thetransmission terminal 16 to the antenna connection terminal 14 serves asa signal transmission side, and a second signal line 34 b from thereception terminal 20 to the antenna connection terminal 14 serves as asignal cutoff side.

Conversely, when the reverse bias voltage Vc2 is applied to the firstcontrol terminal Tc1, turning off the first PIN diode 28 a, and when theforward bias voltage Vc1 is applied to the second control terminal Tc2,turning on the second PIN diode 28 b, the first antenna switch 10A isrepresented by an equivalent circuit shown in FIG. 6 wherein only thereception terminal 20 is connected to the antenna connection terminal 14at high frequencies. A reception signal Sb received by the antenna isthus supplied to the antenna connection terminal 14 and output from thereception terminal 20. In other words, the first signal line 34 a fromthe transmission terminal 16 to the antenna connection terminal 14serves as a signal cutoff side, and the second signal line 34 b from thereception terminal 20 to the antenna connection terminal 14 serves as asignal transmission side.

If the first parallel resonant circuit 26 a is dispensed with and onlythe first PIN diode 28 a is connected, then the first switch circuit 22a is not represented by the equivalent circuit shown in FIG. 3B in thevicinity of the central frequency fo when the PIN diode 28 a is turnedoff, but the parasitic capacitance Cf remains, as shown in FIG. 2B,shifting the resonant frequency into a low frequency range. As a result,the phase characteristic of the first λ/4 transmission line 24 a suffersan error, thereby causing a loss.

With the first antenna switch 10A, the constant of the first inductor 30a of the first parallel resonant circuit 26 a is adjusted to equalizethe resonant frequency of the first parallel resonant circuit 26 a atthe time the first PIN diode 28 a is turned off with the centralfrequency fo of the first antenna switch 10A. Similarly, the constant ofthe second inductor 30 b of the second parallel resonant circuit 26 b isadjusted to equalize the resonant frequency of the second parallelresonant circuit 26 b at the time the second PIN diode 28 b is turnedoff with the central frequency fo of the first antenna switch 10A.

Since the ON resistance Ro of the PIN diode is expressed as Ro<<2πfoLa,only the ON resistance Ro is connected to GND of the first λ/4transmission line 24 a when the first PIN diode 28 a is turned on, andonly the parallel resistance Rf is connected to GND of the first λ/4transmission line 24 a when the first PIN diode 28 a is turned off, asshown in FIGS. 3A and 3B. Consequently, the resonant frequencies of thefirst λ/4 transmission line 24 a at the time the first PIN diode 28 a isturned on and off do not deviate from each other.

With the first antenna switch 10A, therefore, the phase characteristicsof the first λ/4 transmission line 24 a and the second λ/4 transmissionline 24 b do not suffer an error, and the passband at the time theswitch circuits are turned on and the isolation band at the time theswitch circuits are turned off are held in conformity with each other.In other words, the first antenna switch 10A is capable of appropriatelyminimizing the insertion loss caused when the switch circuits are turnedon and maximizing the isolation provided when the switch circuits areturned off in a band that is used by the antenna switch. As a result,the loss of a transmission signal caused in the switch circuits isreduced, and an appropriate amount of attenuation at the time the switchcircuits are turned off is secured.

Modifications of the first antenna switch 10A will be described belowwith reference to FIGS. 7 and 8.

As shown in FIG. 7, an antenna switch 10Aa according to a firstmodification resides in that respective characteristic impedances Zo2 ofthe first λ/4 transmission line 24 a and the second λ/4 transmissionline 24 b are lower than respective characteristic impedances Zo1 (e.g.,50 ohms) of the first λ/4 signal transmission line 18 a and the secondλ/4 signal transmission line 18 b (Zo1>Zo2).

Operation of the antenna switch 10Aa can be explained based on the aboveequation (e).

When the first PIN diode 28 a, for example, is turned off, if theparallel resistance Rf=Z(L)=10 k ohms and the characteristic impedanceZo of the first λ/4 transmission line 24 a=50 ohms, then the impedanceZ(0) at the end on the first signal line side of the first λ/4transmission line 24 a is Z(0)=0.25 ohm.

Since the characteristic impedance of the first λ/4 transmission line 24a of the antenna switch 10Aa is lower than the characteristic impedance(in this case, 50 ohms) of the first λ/4 signal transmission line 18 a,when the characteristic impedance Zo of the first λ/4 transmission line24 a is Zo=25 ohms, the impedance Z(0) at the end on the first signalline side is Z(0)=0.0625 ohm.

The impedance at the end on the first signal line 34 a side of the firstλ/4 transmission line 24 a is smaller than when the characteristicimpedance Zo2 of the first λ/4 transmission line 24 a and thecharacteristic impedance Zo1 of the first λ/4 signal transmission line18 a are the same as each other. The switch circuit thus approaches anideal short-circuited state.

With the antenna switch 10Aa, therefore, the isolation at the time thefirst PIN diode 28 a and the second PIN diode 28 b are turned off, inparticular, the isolation between the antenna connection terminal 14 andthe transmission terminal 16 or the isolation between the antennaconnection terminal 14 and the reception terminal 20 is expanded forefficiently cutting off a reception signal upon transmission and atransmission signal upon reception.

As shown in FIG. 8, an antenna switch 10Ab according to a secondmodification resides in that respective characteristic impedances Zo2 ofthe first λ/4 transmission line 24 a and the second λ/4 transmissionline 24 b are higher than respective characteristic impedances Zo1(e.g., 50 ohms) of the first λ/4 signal transmission line 18 a and thesecond λ/4 signal transmission line 18 b (Zo1<Zo2).

Operation of the antenna switch 10Ab can also be explained based on theabove equation (e).

When the first PIN diode 28 a, for example, is turned on, if the ONresistance Ro=Z(L)=1 ohm and the characteristic impedance Zo of thefirst λ/4 transmission line 24 a=50 ohms, then the impedance Z(0) at theend on the first signal line side of the first λ/4 transmission line 24a is Z(0)=2500 ohms.

Since the characteristic impedance of the first λ/4 transmission line 24a of the antenna switch 10Ab is higher than the characteristic impedance(in this case, 50 ohms) of the first λ/4 signal transmission line 18 a,when the characteristic impedance Zo of the first λ/4 transmission line24 a is Zo=100 ohms, the impedance Z(0) at the end on the first signalline side is Z(0)=10000 ohms.

The impedance at the end on the first signal line 34 a side of the firstλ/4 transmission line 24 a is greater than when the characteristicimpedance Zo2 of the first λ/4 transmission line 24 a and thecharacteristic impedance Zo1 of the first λ/4 signal transmission line18 a are the same as each other. The switch circuit thus approaches anideal open state.

The antenna switch 10Ab is thus capable of minimizing the insertion losscaused when the first PIN diode 28 a and the second PIN diode 28 b areturned on, in particular, the insertion loss between the antennaconnection terminal 14 and the transmission terminal 16 or the insertionloss between the antenna connection terminal 14 and the receptionterminal 20, for efficiently transferring a transmission signal and areception signal.

As described above, the antenna switch 10Aa is effective to expand theisolation of the signal lines wherein the PIN diodes are turned off, andthe antenna switch 10Ab is effective to reduce the insertion loss of thesignal lines wherein the PIN diodes are turned on. Which one of theconfigurations is to be employed may be determined depending on demands,specifications, etc.

An antenna switch according to a second embodiment (hereinafter referredto as a second antenna switch 10B) will be described below withreference to FIG. 9.

As shown in FIG. 9, the second antenna switch 10B is of a configurationsubstantially similar to the first antenna switch 10A described above,but is different therefrom as follows:

Two first λ/4 signal transmission lines 18 a are connected between theantenna connection terminal 14 and the transmission terminal 16, and twosecond λ/4 signal transmission lines 18 b are connected between theantenna connection terminal 14 and the reception terminal 20.

First switch circuits 22 a are connected in association with therespective first λ/4 signal transmission lines 18 a, and similarly,second switch circuits 22 b are connected in association with therespective second λ/4 signal transmission lines 18 b.

Furthermore, the first parallel resonant circuit 26 a of each of thefirst switch circuits 22 a has a plurality of parallel first PIN diodes28 a, and the second parallel resonant circuit 26 b of each of thesecond switch circuits 22 b has a plurality of parallel second PINdiodes 28 b.

In this case also, the constant of the first inductor 30 a of the firstparallel resonant circuit 26 a is adjusted to equalize the resonantfrequency of the first parallel resonant circuit 26 a at the time thefirst PIN diode 28 a is turned off with the central frequency of thesecond antenna switch 10B. Similarly, the constant of the secondinductor 30 b of the second parallel resonant circuit 26 b is adjustedto equalize the resonant frequency of the second parallel resonantcircuit 26 b at the time the second PIN diode 28 b is turned off withthe central frequency of the second antenna switch 10B.

When the first switch circuits 22 a are turned on, i.e., when all thefirst PIN diodes are turned on, the resistance between the firstjunctions a1 and GND is represented by a resistance which is lower thanone ON resistance. As can be understood from the equation (e) above, theimpedance at the end on the first signal line 34 a side of the first λ/4transmission line 24 a is an impedance higher than with one ONresistance. The switch circuits thus approach an ideal open state.

Conversely, when the first switch circuits 22 a are turned off, i.e.,when all the first PIN diodes are turned off, only parallel resistances,which are high, are connected between the first junctions a1 and GND. Ascan be understood from the equation (e) above, the impedance at the endon the first signal line 34 a side of the first λ/4 transmission line 24a is a low impedance depending on the high resistance. In other words,the insertion loss of the switch circuits upon signal transmission canfurther be reduced.

The second antenna switch 10B may employ the same configuration as theantenna switch 10Aa and the antenna switch 10Ab.

In the above embodiment, the two first λ/4 signal transmission lines 18a are connected in series to the first signal line 34 a, and the twosecond λ/4 signal transmission lines 18 b are connected in series to thesecond signal line 34 b. Alternatively, three or more first λ/4 signaltransmission lines 18 a may be connected in series to the first signalline 34 a, and three or more second λ/4 signal transmission lines 18 bmay be connected in series to the second signal line 34 b.

If switch circuits are provided in multiple stages as described above,then the parallel resonant circuits may be dispensed with except for atleast one switch circuit connected to the first signal line 34 a and atleast one switch circuit connected to the second signal line 34 b. Inthe switch circuits where the parallel resonant circuits are dispensedwith, the phase characteristic of the λ/4 transmission line suffers anerror. However, the loss can be reduced by adjusting the characteristicimpedance of the λ/4 transmission line, and the circuits can besimplified. Which one of the configurations is to be employed may bedetermined depending on demands, specifications, etc.

In the above embodiments, the central frequency fo of the operatingfrequency band has mainly been described. Actually, the above advantagesare offered at each of the frequencies contained in the operatingfrequency band.

As shown in FIG. 10, an antenna switch according to a third embodiment(hereinafter referred to as a third antenna switch 10C) is of aconfiguration substantially similar to the first antenna switch 10Adescribed above, but is different therefrom in that it has a third λ/4signal transmission line 18 c connected between the second λ/4 signaltransmission line 18 b and the reception terminal 20, and a third switchcircuit 22 c connected parallel to the third λ/4 signal transmissionline 18 c. Capacitors C4, C5 are connected in series respectivelybetween the second λ/4 signal transmission line 18 b and the third λ/4signal transmission line 18 c and between the third λ/4 signaltransmission line 18 c and the reception terminal 20. As with thecapacitors C1 through C3, the capacitors C4, C5 are capacitors forblocking currents for turning on and off PIN diodes, to be describedlater, and operate as a short circuit at high frequencies.

As with the second switch circuit 22 b, the third switch circuit 22 c isconnected between a signal line between the third λ/4 signaltransmission line 18 c and the capacitor C5 and GND (ground). The thirdswitch circuit 22 c comprises a series-connected circuit of a third λ/4transmission line 24 c and a third parallel resonant circuit 26 c whichare connected in series to each other at a third junction a3.

The third parallel resonant circuit 26 c comprises a third PIN diode 28c connected between the third junction a3 and GND, a third inductor 30 cconnected between the third junction a3 and a second control terminalTc2, and a third capacitor Cc connected between the second controlterminal Tc2 and GND. The third capacitor Cc operates as a capacitor forblocking currents for turning on and off the third PIN diode 28 c.

The third switch circuit 22 c also includes a series-connected circuitof a resistor Rr for forming a reception terminating resistance and acapacitor Cr, connected parallel to the third PIN diode 28 c. Thecapacitor Cr operates as a capacitor for blocking currents for turningon and off the third PIN diode 28 c.

To the second control terminals Tc2, there are applied the forward biasvoltage Vc1 for passing a forward current through the second PIN diode28 b and the third PIN diode 28 c to turn on the second PIN diode 28 band the third PIN diode 28 c and the reverse bias voltage Vc2 forreversely biasing the second PIN diode 28 b and the third PIN diode 28 cto turn off the second PIN diode 28 b and the third PIN diode 28 c. Whenthe forward bias voltage Vc1 is applied to the first control terminalTc1, the reverse bias voltage Vc2 is applied to the second controlterminals Tc2. When the reverse bias voltage Vc2 is applied to the firstcontrol terminal Tc1, the forward bias voltage Vc1 is applied to thesecond control terminals Tc2. The reverse bias voltage Vc2 which isapplied to the first control terminal Tc1 and the reverse bias voltageVc2 which is applied to the second control terminals Tc2 may havedifferent voltage levels.

Circuit operation of the third antenna switch 10C will be describedbelow with reference to FIGS. 11A through 12. Since operation of thefirst switch circuit 22 a and the second switch circuit 22 b has beendescribed above, the third switch circuit 22 c will primarily bedescribed below.

When the forward bias voltage Vc1 is applied to the second controlterminal Tc2, the third PIN diode 28 c is turned on. At this time, thethird switch circuit 22 c is represented by an equivalent circuit shownin FIG. 11A. Specifically, a circuit comprising an inductance La, an ONresistance Ro of the third PIN diode 28 c, and the resistor Rr forforming a reception terminating resistance which are connected parallelto each other is connected in series between the third λ/4 transmissionline 24 c and GND.

Conversely, when the reverse bias voltage Vc2 is applied to the secondcontrol terminal Tc2, the third PIN diode 28 c is turned off. At thistime, the third switch circuit 22 c is represented by an equivalentcircuit shown in FIG. 11B. Specifically, a parallel resonant circuitcomprising an inductance La, a parasitic capacitance Cf due to thedepletion layer of the third PIN diode 28 c, a parallel resistance Rf ofthe third PIN diode 28 c, and the resistor Rr for forming a receptionterminating resistance which are connected parallel to each other isconnected in series between the third λ/4 transmission line 24 c andGND.

In this case, the inductance La also has a value established such thatthe central frequency fo of the first antenna switch 10A and theresonant frequency of the parallel resonant circuit that is made up ofthe parasitic capacitance Cf, the parallel resistance Rf, and theinductance La are in agreement with each other.

As described above, the third switch circuit 22 c is of a configurationincluding the parallel-connected resistor Rr for forming a receptionterminating resistance. Since the ON resistance Ro and the resistor Rrhave a magnitude relationship of Ro<<Rr, the resistor Rr does not affectthe operation of the third switch circuit 22 c when the third PIN diode28 c is turned on. Since the parallel resistance Rf and the resistor Rrhave a magnitude relationship of Rf>>Rr, the impedance on the signalline side is determined by the resistor Rr.

Specifically, if the characteristic impedance of the third λ/4transmission line 24 c is of 50 ohms and the resistor Rr for forming areception terminating resistance is of 50 ohms, then the combinedresistance (Rf//Rr) of the parallel resistance Rf (e.g., 10 k ohms) andthe resistor Rr is of 49.751 ohms. The impedance of the third λ/4transmission line 24 c on the signal line side is terminated with50×50/49.751=50.250 ohms according to the equation (e) (the terminatingresistance is of 50.250 ohms). Actually, the value of the resistor Rr isdetermined so that the terminating resistance is of 50 ohms, forexample.

When the third PIN diode 28 c is turned on, if the ON resistance Ro=1ohm, then since the combined resistance (Ro//Rr) of the ON resistance Roand the resistor Rr is of 0.9804 ohm, the impedance of the third λ/4transmission line 24 c on the signal line side is of 50×50/0.9804=2550ohms according to the equation (e).

Therefore, when the forward bias voltage Vc1 is applied to the firstcontrol terminal Tc1, turning on the first PIN diode 28 a, and thereverse bias voltage Vc2 is applied to the second control terminal Tc2,turning off the second PIN diode 28 b and the third PIN diode 28 c, thethird antenna switch 10C is represented by an equivalent circuit shownin FIG. 12 wherein only the transmission terminal 16 is connected to theantenna connection terminal 14 at high frequencies, and a terminatingresistor Re of 50 ohms, for example, is connected to the receptionterminal 20. A transmission signal Sa supplied to the transmissionterminal 16 is thus transmitted via the antenna connection terminal 14.In other words, a first signal line 34 a from the transmission terminal16 to the antenna connection terminal 14 serves as a signal transmissionside, and a second signal line 34 b from the reception terminal 20 tothe antenna connection terminal 14 serves as a signal cutoff side.

If the third switch circuit 22 c were not present, then the impedance ofthe second λ/4 transmission line 24 b on the signal line side would beof a small value, and the signal line is ideally in a short-circuitedstate, as described above. In other words, since the impedance on thereceiver side when the switch is turned off is of 0 ohm, resulting intotal reflection, the reception amplifier connected to the receptionterminal 20 may become unstable in operation.

Inasmuch as the third antenna switch 10C includes the third switchcircuit 22 c, the impedance on the receiver side when the switch isturned off is of the value of the terminating resistor Re, e.g., 50ohms, thereby allowing the third antenna switch 10C to achieve impedancematching with other circuits. Therefore, the reception amplifierconnected to the reception terminal 20 is rendered stable in operation.

Conversely, when the reverse bias voltage Vc2 is applied to the firstcontrol terminal Tc1, turning off the first PIN diode 28 a, and theforward bias voltage Vc1 is applied to the second control terminal Tc2,turning on the second PIN diode 28 b and the third PIN diode 28 c, thethird antenna switch 10C is represented by the equivalent circuit shownin FIG. 6 wherein only the reception terminal 20 is connected to theantenna connection terminal 14 at high frequencies, and a receptionsignal Sb received by the antenna is thus supplied to the antennaconnection terminal 14 and output from the reception terminal 20. Inother words, the first signal line 34 a from the transmission terminal16 to the antenna connection terminal 14 serves as a signal cutoff side,and the second signal line 34 b from the reception terminal 20 to theantenna connection terminal 14 serves as a signal transmission side.Therefore, the resistor Rr does not affect reception of the signal.

Modifications of the third antenna switch 10C will be described belowwith reference to FIGS. 13 and 14.

As shown in FIG. 13, an antenna switch 10Ca according to a firstmodification resides in that respective characteristic impedances Zo2 ofthe first λ/4 transmission line 24 a through third λ/4 transmission line24 c are lower than respective characteristic impedances Zo1 (e.g., 50ohms) of the first λ/4 signal transmission line 18 a through third λ/4signal transmission line 18 c (Zo1>Zo2).

With the antenna switch 10Ca, as with the antenna switch 10Aa, theimpedance at the end on the first signal line 34 a side of the first λ/4transmission line 24 a is smaller than when the characteristic impedanceZo2 of the first λ/4 transmission line 24 a and the characteristicimpedance Zo1 of the first λ/4 signal transmission line 18 a are thesame as each other. The switch circuit thus approaches an idealshort-circuited state.

With the antenna switch 10Ca, therefore, the isolation at the time thefirst PIN diode 28 a through the third PIN diode 28 c are turned off, inparticular, the isolation between the antenna connection terminal 14 andthe transmission terminal 16 or the isolation between the antennaconnection terminal 14 and the reception terminal 20 is expanded forefficiently cutting off a reception signal upon transmission and atransmission signal upon reception. Moreover, the impedance on thereceiver side when the switch is turned off is made closer to the idealvalue of the terminating resistor Re, e.g., 50 ohms, thereby making thereception amplifier connected to the reception terminal 20 more stablein operation.

As shown in FIG. 14, an antenna switch 10Cb according to a secondmodification resides in that respective characteristic impedances Zo2 ofthe first λ/4 transmission line 24 a through third λ/4 transmission line24 c are higher than respective characteristic impedances Zo1 (e.g., 50ohms) of the first λ/4 signal transmission line 18 a through third λ/4signal transmission line 18 c (Zo1<Zo2).

With the antenna switch 10Cb, as with the antenna switch 10Ab, theimpedance at the end on the first signal line 34 a side of the first λ/4transmission line 24 a is greater than when the characteristic impedanceZo2 of the first λ/4 transmission line 24 a and the characteristicimpedance Zo1 of the first λ/4 signal transmission line 18 a are thesame as each other. The switch circuit thus approaches an ideal openstate.

The antenna switch 10Cb is thus capable of minimizing the insertion losscaused when the first PIN diode 28 a through the third PIN diode 28 care turned on, in particular, the insertion loss between the antennaconnection terminal 14 and the transmission terminal 16 or the insertionloss between the antenna connection terminal 14 and the receptionterminal 20, for efficiently transferring a transmission signal and areception signal. Moreover, the impedance on the receiver side when theswitch is turned off is made closer to the ideal value of theterminating resistor Re, e.g., 50 ohms, thereby making the receptionamplifier connected to the reception terminal 20 more stable inoperation.

As described above, the antenna switch 10Ca is effective to expand theisolation of the signal lines wherein the PIN diodes are turned off, andthe antenna switch 10Cb is effective to reduce the insertion loss of thesignal lines wherein the PIN diodes are turned on. Which one of theconfigurations is to be employed may be determined depending on demands,specifications, etc.

An antenna switch according to a fourth embodiment (hereinafter referredto as a fourth antenna switch 10D) will be described below withreference to FIG. 15.

The fourth antenna switch 10D is of a configuration which issubstantially similar to the third antenna switch 10C described above,but is different therefrom as follows:

Two first λ/4 signal transmission lines 18 a are connected between anantenna connection terminal 14 and a transmission terminal 16, and twosecond λ/4 signal transmission lines 18 b and a third λ/4 signaltransmission line 18 c are connected between the antenna connectionterminal 14 and a reception terminal 20.

First switch circuits 22 a are connected in association with therespective first λ/4 signal transmission lines 18 a, and, similarly,second switch circuits 22 b are connected in association with therespective second λ/4 signal transmission lines 18 b and a third switchcircuit 22 c is connected in association with the third λ/4 signaltransmission line 18 c.

Furthermore, the first parallel resonant circuit 26 a of each of thefirst switch circuits 22 a has a plurality of parallel first PIN diodes28 a, the second parallel resonant circuit 26 b of each of the secondswitch circuits 22 b has a plurality of parallel second PIN diodes 28 b,and the third parallel resonant circuit 26 c of the third switch circuit22 c has a plurality of parallel third PIN diodes 28 c.

In this case also, the constant of the first inductor 30 a of the firstparallel resonant circuit 26 a is adjusted to equalize the resonantfrequency of the first parallel resonant circuit 26 a at the time thefirst PIN diode 28 a is turned off with the central frequency of thesecond antenna switch 10B. Similarly, the constants of the secondinductor 30 b of the second parallel resonant circuit 26 b and the thirdinductor 30 c of the third parallel resonant circuit 26 c are adjustedto equalize the resonant frequencies of the second parallel resonantcircuit 26 b and the third parallel resonant circuit 26 c at the timethe second PIN diode 28 b and the third PIN diode 28 c are turned offwith the central frequency of the fourth antenna switch 10D.

When the first switch circuits 22 a are turned on, i.e., when all thefirst PIN diodes 28 a are turned on, the resistance between the firstjunctions a1 and GND is represented by a resistance which is lower thanone ON resistance. As can be understood from the equation (e) above, theimpedance at the end on the first signal line 34 a side of the first λ/4transmission line 24 a is an impedance higher than with one ONresistance. The switch circuits thus approach an ideal open state.

Conversely, when the first switch circuits 22 a are turned off, i.e.,when all the first PIN diodes 28 a are turned off, only parallelresistances, which are high, are connected between the first junctionsa1 and GND. As can be understood from the equation (e) above, theimpedance at the end on the first signal line 34 a side of the first λ/4transmission line 24 a is a low impedance depending on the highresistance. In other words, the insertion loss of the switch circuitsupon signal transmission can further be reduced.

The fourth antenna switch 10D may employ the same configuration as theantenna switch 10Ca and the antenna switch 10Cb.

In the above embodiment, the two first λ/4 signal transmission lines 18a are connected in series to the first signal line 34 a, and the twosecond λ/4 signal transmission lines 18 b are connected in series to thesecond signal line 34 b. Alternatively, three or more first λ/4 signaltransmission lines 18 a may be connected in series to the first signalline 34 a, and three or more second λ/4 signal transmission lines 18 bmay be connected in series to the second signal line 34 b.

If switch circuits are provided in multiple stages as described above,then the parallel resonant circuits may be dispensed with except for atleast one switch circuit connected to the first signal line 34 a and atleast one switch circuit connected to the second signal line 34 b. Inthe switch circuits where the parallel resonant circuits are dispensedwith, the phase characteristic of the λ/4 transmission line suffers anerror. However, the loss can be reduced by adjusting the characteristicimpedance of the λ/4 transmission line, and the circuits can besimplified. Which one of the configurations is to be employed may bedetermined depending on demands, specifications, etc.

An antenna switch according to a fifth embodiment (hereinafter referredto as a fifth antenna switch 10E) will be described below with referenceto FIG. 16.

The fifth antenna switch 10E is of a configuration which issubstantially similar to the third antenna switch 10C described above,but is different therefrom as follows:

The fifth antenna switch 10E has a fourth λ/4 signal transmission line18 d connected between the first λ/4 signal transmission line 18 a andthe transmission terminal 16 and a fourth switch circuit 22 d connectedparallel to the fourth λ/4 signal transmission line 18 d.

The fourth switch circuit 22 d is connected between a signal linebetween the fourth λ/4 signal transmission line 18 d and a capacitor C1and GND (ground). The fourth switch circuit 22 d comprises aseries-connected circuit of a fourth λ/4 transmission line 24 d and afourth parallel resonant circuit 26 d which are connected in series toeach other at a fourth junction a4.

The fourth parallel resonant circuit 26 d comprises a fourth PIN diode28 d connected between the fourth junction a4 and GND, a fourth inductor30 d connected between the fourth junction a4 and a first controlterminal Tc1, and a fourth capacitor Cd connected between the firstcontrol terminal Tc1 and GND. The fourth capacitor Cd operates as acapacitor for blocking currents for turning on and off the fourth PINdiode 28 d.

The fourth switch circuit 22 d also includes a series-connected circuitof a resistor Rt for forming a transmission terminating resistance and acapacitor Ct, which is connected parallel to the fourth PIN diode 28 d.

The fourth switch circuit 22 d is thus of a configuration identical tothe third switch circuit 22 c on the receiver side.

Therefore, when the forward bias voltage Vc1 is applied to the firstcontrol terminal Tc1, turning on the first PIN diode 28 a and the fourthPIN diode 28 d, and the reverse bias voltage Vc2 is applied to thesecond control terminal Tc2, turning off the second PIN diode 28 b andthe third PIN diode 28 c, the fifth antenna switch 10E is represented bythe equivalent circuit shown in FIG. 12 wherein only the transmissionterminal 16 is connected to the antenna connection terminal 14 at highfrequencies, and a terminating resistor of 50 ohms, for example, isconnected to the reception terminal 20. In this case, the impedance onthe receiver side when the switch is turned off is of the value of theterminating resistor Re, e.g., 50 ohms, thereby allowing the fifthantenna switch 10E to achieve impedance matching with other circuits.Therefore, the reception amplifier connected to the reception terminal20 is rendered stable in operation.

Conversely, when the reverse bias voltage Vc2 is applied to the firstcontrol terminal Tc1, turning off the first PIN diode 28 a and thefourth PIN diode 28 d, and the forward bias voltage Vc1 is applied tothe second control terminal Tc2, turning on the second PIN diode 28 band the third PIN diode 28 c, the fifth antenna switch 10E isrepresented by an equivalent circuit shown in FIG. 17 wherein only thereception terminal 20 is connected to the antenna connection terminal 14at high frequencies, and a terminating resistor Re of, for example, 50ohms is connected to the transmission terminal 16. In this case, theimpedance on the transmitter side when the switch is turned off is ofthe value of the terminating resistor Re, e.g., 50 ohms, therebyallowing the fifth antenna switch 10E to achieve impedance matching withother circuits.

The fifth antenna switch 10E may employ the same configuration as theantenna switch 10Ca, the antenna switch 10Cb, and the fourth antennaswitch 10D.

In the above embodiments, the central frequency fo of the operatingfrequency band has mainly been described. Actually, the above advantagesare offered at each of the frequencies contained in the operatingfrequency band.

An antenna switch according to a sixth embodiment (hereinafter referredto as a sixth antenna switch 10F) will be described below with referenceto FIG. 18.

The sixth antenna switch 10F is of a configuration which issubstantially similar to the third antenna switch 10C described above,but has a first switch circuit 22 a through a third switch circuit 22 cwhich are different therefrom in configuration as follows:

The first switch circuit 22 a comprises a series-connected circuit of afirst PIN diode 28 a and a first capacitor Ca, connected between a firstλ/4 transmission line 24 a and GND, and a first control terminal Tc1connected to the junction between the first PIN diode 28 a and the firstcapacitor Ca.

The second switch circuit 22 b comprises a series-connected circuit of asecond PIN diode 28 b and a second capacitor Cb, connected between asecond transmission line 24 b and GND, and a second control terminal Tc2connected to the junction between the second PIN diode 28 b and thesecond capacitor Cb.

The third switch circuit 22 c comprises a series-connected circuit of athird PIN diode 28 c and a third capacitor Cc, connected between a thirdλ/4 transmission line 24 c and GND, a second control terminal Tc2connected to the junction between the third PIN diode 28 c and the thirdcapacitor Cc, and a resistor Rr for forming a reception terminatingresistance, connected between the cathode of the third PIN diode 28 cand GND.

Therefore, when the forward bias voltage Vc1 is applied to the firstcontrol terminal Tc1, turning on the first PIN diode 28 a, and thereverse bias voltage Vc2 is applied to the second control terminal Tc2,turning off the second PIN diode 28 b and the third PIN diode 28 c, thesixth antenna switch 10F is represented by the equivalent circuit shownin FIG. 12 wherein only the transmission terminal 16 is connected to theantenna connection terminal 14 at high frequencies, and a terminatingresistor Re of, for example, 50 ohms is connected to the receptionterminal 20. In this case, the impedance on the receiver side when theswitch is turned off is of the value of the terminating resistor Re,e.g., 50 ohms, thereby allowing the sixth antenna switch 10F to achieveimpedance matching with other circuits. Therefore, the receptionamplifier connected to the reception terminal 20 is rendered stable inoperation.

Conversely, when the reverse bias voltage Vc2 is applied to the firstcontrol terminal Tc1, turning off the first PIN diode 28 a, and theforward bias voltage Vc1 is applied to the second control terminal Tc2,turning on the second PIN diode 28 b and the third PIN diode 28 c, thesixth antenna switch 10F is represented by the equivalent circuit shownin FIG. 6 wherein only the reception terminal 20 is connected to theantenna connection terminal 14 at high frequencies.

The equivalent circuit of the sixth antenna switch 10F in the vicinityof the central frequency fo when the first PIN diode 28 a is turned off,is not the same as shown in FIG. 3B, but includes a parasiticcapacitance Cf which remains as shown in FIG. 2B, thereby shifting theresonant frequency into a low frequency range. Thus, the sixth antennaswitch 10F is poorer in performance than the third antenna switch 10C.However, since the sixth antenna switch 10F is structurally simple, itis effective in applications where small size and lower cost arepreferable to performance.

The sixth antenna switch 10F may employ the same configuration as theantenna switch 10Ca, the antenna switch 10Cb, the fourth antenna switch10D, and the fifth antenna switch 10E.

The high frequency switch according to the present invention is notlimited to the above embodiments, but may adopt various configurationswithout departing from the scope of the invention.

1. A high frequency switch including a first switch circuit connectedparallel to a first λ/4 signal transmission line for transmitting signalfrom a transmission terminal, the first switch circuit comprising afirst λ/4 transmission line and a circuit including one or more firstPIN diode, the first λ/4 transmission line and the circuit beingconnected in series to each other, and a second switch circuit connectedparallel to a second λ/4 signal transmission line for receiving signalby a reception terminal, the second switch circuit comprising a secondλ/4 transmission line and a circuit including one or more second PINdiode, the second λ/4 transmission line and the circuit being connectedin series to each other, the high frequency switch comprising: a thirdswitch circuit connected parallel to a third λ/4 signal transmissionline connected at least between the reception terminal and the secondλ/4 signal transmission line, the third switch circuit comprising athird λ/4 transmission line and a circuit including one or more thirdPIN diode, the third λ/4 transmission line and the circuit beingconnected in series to each other; and a resistor for forming aterminating resistance, the resistor connected parallel to the third PINdiode.
 2. A high frequency switch according to claim 1, comprising: afourth switch circuit connected parallel to a fourth λ/4 signaltransmission line connected between the transmission terminal and thefirst λ/4 signal transmission line, the fourth switch circuit comprisinga fourth λ/4 transmission line and a circuit including one or morefourth PIN diode, the fourth λ/4 transmission line and the circuit beingconnected in series to each other; and a resistor for forming aterminating resistance, the resistor connected parallel to the fourthPIN diode.
 3. A high frequency switch according to claim 1, wherein thehigh frequency switch has an operating frequency band with a centralfrequency fo and a wavelength λ corresponding to the central frequencyfo; the first switch circuit comprises the first λ/4 transmission lineand a parallel resonant circuit including the one or more first PINdiode, the first λ/4 transmission line and the parallel resonant circuitbeing connected in series to each other; the second switch circuitcomprises the second λ/4 transmission line and a parallel resonantcircuit including the one or more second PIN diode, the second λ/4transmission line and the parallel resonant circuit being connected inseries to each other; and the third switch circuit comprises the thirdλ/4 transmission line and a parallel resonant circuit including the oneor more third PIN diode, the third λ/4 transmission line and theparallel resonant circuit being connected in series to each other; eachof the parallel resonant circuits having a constant established suchthat the resonant frequency thereof when the corresponding one of thePIN diodes is turned off is the same as the central frequency fo.
 4. Ahigh frequency switch according to claim 2, wherein the high frequencyswitch has an operating frequency band with a central frequency fo and awavelength λ corresponding to the central frequency fo; the first switchcircuit comprises the first λ/4 transmission line and a parallelresonant circuit including the one or more first PIN diode, the firstλ/4 transmission line and the parallel resonant circuit being connectedin series to each other; the second switch circuit comprises the secondλ/4 transmission line and a parallel resonant circuit including the oneor more second PIN diode, the second λ/4 transmission line and theparallel resonant circuit being connected in series to each other; thethird switch circuit comprises the third λ/4 transmission line and aparallel resonant circuit including the one or more third PIN diode, thethird λ/4 transmission line and the parallel resonant circuit beingconnected in series to each other; and the fourth switch circuitcomprises the fourth λ/4 transmission line and a parallel resonantcircuit including the one or more fourth PIN diode, the fourth λ/4transmission line and the parallel resonant circuit being connected inseries to each other; each of the parallel resonant circuits having aconstant established such that the resonant frequency thereof when thecorresponding one of the PIN diodes is turned off is the same as thecentral frequency fo.
 5. A high frequency switch according to claim 1,wherein the parallel resonant circuit includes a plurality of PINdiodes.
 6. A high frequency switch according to claim 1, wherein the λ/4transmission lines have a characteristic impedance which is smaller thana characteristic impedance of the λ/4 signal transmission lines.
 7. Ahigh frequency switch according to claim 1, wherein the λ/4 transmissionlines have a characteristic impedance which is greater than acharacteristic impedance of the λ/4 signal transmission lines.